Use the cURL library, with CURLFile to send the file. This example
converts the file test.pdf to XML.
You may need to enable the curl plugin in order for the curl_ functions to work. For example, if you're on a linux based system you might need to install packages relating to php-curl, or on Windows you may need to add extension=php_curl.dll to your php.ini. See Fatal error: Call to undefined function curl_init() using Batch File for more details.